The invention relates to an electronic balance with a housing that can be put under an above-ambient pressure (also referred to as over-pressure). Balances of this kind are used, e.g., for applications in areas where an explosion hazard exists.
Hazardous areas with an increased risk of explosions are characterized by the fact that they contain an atmosphere composed of a mixture of normal ambient air and gases that can be brought to ignition or even explosion by sparks (for example in switches or electrical contacts) or by elevated temperatures.
Protective measures for electrical equipment to allow its use in the presence of an explosion hazard are known under different types according to the nature of the technical solution chosen by the manufacturer, such as:                a pressure-proof encapsulation provided by the housing, where the latter is designed to withstand the explosion and associated pressure of an explosive mixture inside the housing and to prevent it from spreading to the surrounding atmosphere;        an intrinsically safe design of the equipment, meaning that the energy level in the electrical circuits is kept low enough to prevent the occurrence of sparks, arcs or temperature increases of a size that could cause ignition;        a sand encapsulation, where the housing is filled with a fine granular material, so that an arc occurring inside the housing cannot ignite an explosive atmosphere surrounding the housing;        a pressurized encapsulation, where a protective gas such as compressed air is maintained at an above-ambient pressure inside the housing to reduce the extent to which the surrounding atmosphere can enter into the housing.        
A method and apparatus for operating an electrical device in a hazardous atmosphere are described in DE-A-44 18 158, proposing the use of a rinse gas flowing through a housing that is protected according to a concept of pressurized encapsulation. The above-ambient pressure inside the housing can be kept at a desired level by monitoring the rinse gas that flows out of the housing through a gas outlet and by regulating the flow of gas into the housing by means of a proportional valve.
The application of another of the aforementioned types of protection is disclosed in DE-A-34 37 959. This reference proposes an intrinsically safe design for the weighing system, the signal-processing unit, and the indicator unit. The conductive connections are limited to a low voltage, and the power supply unit is accommodated in a separate housing. Current-carrying conductors and plug connections are designed intrinsically safe with regard to explosion hazards.
The European Patent Application EP-A-0 018 656 describes a commercially available type of balance in which the weighing system is enclosed in a pressure-proof housing, where the opening in the housing for the vertical mechanical connecting member is designed as a narrow and relatively long, contact-free passage.
However, as a disadvantage of the last-mentioned concept, since the inside of the housing shares the hazardous atmosphere of the outside environment, the balance can still be destroyed if an explosion is ignited inside the housing, even though the explosion and associated pressure are prevented from spreading to the outside. This drawback can be overcome through the concept of a pressurized encapsulation of the balance, where a constant over-pressure is maintained by the inflow of a gas, e.g., compressed air, which prevents to a large extent that the hazardous explosive atmosphere of the outside environment could enter the inside of the housing.
In JP-2213730, a balance with a Roberval-type weighing mechanism is described which has a housing that can be put under an above-ambient pressure, with a horizontal passage through the housing for the force-transmitting rod member. The passage is provided with a bellows-type seal. The forces caused by the internal pressure act on the enclosed portion of the force-transmitting part as well as on the bellows-type seal, but the upward and downward forces cancel each other, so that pressure fluctuations inside the housing have no effect on the measured values.
The arrangement just described is not suitable for highly sensitive balances such as for example precision balances, not to mention analytical balances where a weighing accuracy in the microgram range may be required, because the bellows-type seal exerts reactive forces—including vertically directed force components—on the force-transmitting rod member, a problem that manifests itself in particular through a hysteresis effect, i.e., a difference in the measured results depending on whether the weighing load on the balance is increasing or decreasing. If one were to choose a thinner material for the bellows in an attempt to reduce the aforementioned forces, the bellows would lose the stability necessary to hold its own weight and maintain its shape, and/or it could bulge as a result of the internal pressure. Added to this problem is the difficulty of maintaining a prescribed constant internal pressure, particularly in the presence of temperature fluctuations.